The present invention relates to the refining of lignocellulosic material, and more particularly, to so-called high consistency pulp refining.
The technology associated with the refining of cellulosic material for paper making purposes, has been developed to a high degree of effectiveness in so-called "flat disc" refiners. In the past, the performance of such machines has been improved by increasing the disc diameter and speed of rotation. Increase in size has, however, reached practical limits with respect to the material integrity of the disc, and the adverse effect of high centrifical forces on the retention time of material as it passes radially outward along the disc. In particular, the high centrifical forces acting on the steam generated during high consistency refining imparts such high radial velocities, as to cause the steam to carry the pulp material out of the grinding space before full refining action has been completed.
Various efforts have been made in recent decades to overcome some of these problems. Developments represented by U.S. Pat. Nos. 4,090,672; 4,238,016; and 4,253,613 provide a combination of a radially extending and inclined refining zones, the latter having a retaining influence on the steam, thereby inhibiting the "blow-out" effect. In other words, substantially the same total refining gap length can be achieved without blow-out, relative to flat disc refiners.
U.S. Pat. No. 4,253,613 discloses a single conical refining zone following a single radial refining zone, which receives wood chips from a single feed screw of solid flight construction. Several deficiencies are inherent to this configuration. Having a radial, or flat refining zone prior to the conical refining zone impedes the free flow of steam backward from the conical refining zone, due to the forward flowing steam generated in the flat refining zone. This forces all the steam in the conical zone to flow forward. The flat zone acts as a pressure seal to prevent steam generated in the conical zone from backflowing into the flat zone. As a result, retention time of the material in the conical zone cannot be controlled by direct adjustment of the steam pressure difference across the conical zone (i.e., by steam flow valves). Because the nature of the fiber development in the conical zone is so dependent on retention time, fine control of retention time would be advantageous. Yet, the machine disclosed in the '613 experiences the inherent reduction in the effectiveness of mechanical conveying along the conical zone, without the ability to control conveyance via steam flow control. Some of the steam generated in the flat zone of the '613 refiner does flow backward. The solid flighted feeding screw does not provide openings for the backflowing of steam while conveying the material to be fed forward. This configuration can result in the material feed being interrupted by the steam flow and cause instability of the refining operation, thereby reducing production rates and pulp quality.
Another high consistency conical refiner is disclosed by U.S. Pat. No. 4,401,280, where the material is fed from a solid flighted feed screw which is cantilevered and operated by a separate drive mechanism. The feed screw feeds into a radial zone consisting of enclosed pockets in the rotating member from which the material is fed into the conical refining zone. The cantilevered feed screw arrangement with a separate drive arrangement operates at a lower rotation speed than the rotating disc of the refining zone due to mechanical and critical speed limitations. This results in lower centrifugal force in the feeder, limiting steam and fiber separation. The limitations of the solid feeder flight arrangement as indicated above with respect to U.S. Pat. No. 4,253,613, are also present. The substantially radial feed pockets have both walls rotating with the disc and therefore will provide no breaking action to chips that are being fed since there are no relative rotating bars to provide the breaking. This arrangement will feed the chips directly to the conical refining zone and require the chip breaking to occur there. This results in a substantial reduction in the fiber refining area in the conical zone. This would also potentially limit the machine to applications where chips must be refined into a coarse fiber prior to delivery into the refiner machine. Transfer of material to be refined from the feed screw to the rotating feed pockets is also difficult because the rotating pockets and feed screw are not physically attached. Since the conical refining zone gap is adjusted by moving the rotating disc, space must be provided between the two members. This increases the possibility that backflowing steam will interrupt the continuous feed of material.
The concept of a two zone, high consistency conical refiner has been disclosed in U.S. Pat. No. 5,127,591. This patent discloses a single material feeding location at the center and transverse to the rotation axis of the machine. The conical refining zones extend with the major diameter away from the center and receive a predominantly axial feed of material. The deficiencies in this concept arise from the predominantly axial feeding into the conical refiner zones, which does not allow centrifugal force to feed the material into the refining zone, particularly against the backflowing steam generated in the conical zones. Therefore, in order to feed fibrous material into the conical refining zones, all of the generated steam must flow forward. Flowing all the steam forward requires a higher pressure at the grinding space inlet, than at any point along the conical plate surface or the discharge. Since steam pressures as high as 50 to 100 psi above the machine operating pressure can be generated in the refining zone, the complete inlet area of the machine back to the pressure seal where chips are fed into the refiner must be maintained at this high pressure. This high pressure and resulting high temperature tends to darken the fibrous material, resulting in unacceptable darkening of the final sheet of paper to be produced. Also, forcing all the steam to flow forward through the conical refining zone reduces the retention time of the material in the zone, thereby increasing the intensity of the refining action. This limits the range of pulp characteristics that can be achieved in a given refiner. The high inlet pressure required to feed all the steam forward and convey the material to be refined into the refining zone also requires that the inlet zone of the refiner and the equipment feeding the refiner, be capable of withstanding this higher pressure. Thus heavier construction and increased manufacturing cost result.